The present invention relates generally to radio frequency communication systems and, more particularly, to receiver configurations therefor.
Radio frequency (RF) receivers for cellular telephone base stations and other telecommunication system components must provide high degrees of both selectivity (the ability to distinguish between signals separated by small frequency differences) and sensitivity (the ability to receive weak signals). Typically, an incoming RF signal is first passed through a low loss, passive, RF bandpass filter to remove signal components outside of the frequency range of the desired signal. The resulting filtered signal is then usually amplified by an amplifier that does not introduce a significant amount of noise (i.e., a low noise amplifier or LNA). In this manner, the LNA and other system components are protected from any amplified, undesired signals.
The advent of widespread cellular telephone communication systems has increased the demands placed on these RF filter-amplifier configurations. Selectivity has been increased by using two identical RF bandpass filters instead of relying on a single filter. See U.S. Pat. Nos. 5,537,680, 5,412,339, and 5,355,524. Additional selectivity has also been realized by manufacturing filters having more sections, which add more poles (i.e., frequencies at which the transfer function of a filter approaches unity (0 dB) and the reflection coefficient approaches zero). However, adding another filter or more sections comes at the expense of increased losses (i.e., decreased sensitivity) and, therefore, increased attenuation of potentially very weak incoming signals.
RF filters have been designed such that each additional section introduces very little signal loss. For example, RF filters have included resonant elements utilizing high temperature superconducting (HTSC) materials. HTSC filters have been shown to provide quality factors (Q, the ratio of the center frequency to the 3 dB bandwidth) as high as 100,000. In general, however, low loss filters have a quality factor above about 12,000 and preferably above about 20,000. With each additional pole of an HTSC filter introducing negligible losses, demands for still further rejection have resulted in filter designs having more and more poles. HTSC filters, however, are costly relative to RF filters utilizing conventional materials, as HTSC materials are relatively costly to manufacture and must be maintained at very low temperatures.
While high-order (e.g., sixteen poles) HTSC filters have been manufactured, filters with more than about sixteen sectionsxe2x80x94whether HTSC or conventional filtersxe2x80x94become impractical for several reasons, including the feasibility of manufacturing and tuning them. In general, tuning a filter requires adjustment of both the resonant frequency of each resonant section and the degree and type of electromagnetic coupling between sections. Tuning these high-order filters is problematic because the filter""s response becomes highly sensitive to even minor changes in component parameter values.
In accordance with one aspect of the present invention, an RF receiver includes a first RF filter stage, an amplifier stage, and a second RF filter stage having a different selectivity than the first filter stage. An output terminal of the first RF filter stage is coupled to an input terminal of the amplifier stage, while an output terminal of the amplifier stage is coupled to an input terminal of the second RF filter stage.
The second RF filter stage may provide more rejection than the first RF filter stage, which may include a low loss RF filter. The low loss RF filter of the first RF filter stage may rely on superconducting resonant elements. In contrast, the second RF filter stage may include conventional resonant elements.
According to another aspect of the present invention, an RF receiver includes a first RF filter stage, an amplifier stage, and a second RF filter stage. An output terminal of the first RF filter stage is coupled to an input terminal of the amplifier stage, while an output terminal of the amplifier stage is coupled to an input terminal of the second RF filter stage. The first RF filter stage includes a low loss RF filter, and the second RF filter stage includes an RF filter having a higher insertion loss than the low loss RF filter.
According to yet another aspect of the present invention, an RF receiver includes a first RF filter stage including superconducting material and having an output terminal. The RF receiver further includes an amplifier having an input terminal coupled to the output terminal of the first RF filter stage. The RF receiver still further includes a second RF filter stage including superconducting material and having an input terminal coupled to an output terminal of the amplifier. Both the first and second RF filter stages include high-order RF filters.
According to still another aspect of the present invention, an RF receiver includes a first RF filter, a second RF filter coupled to the first RF filter, an amplifier coupled to the second RF filter, a third RF filter coupled to the amplifier, and a fourth RF filter coupled to the third RF filter.
The first through fourth RF filters may include thin film superconducting elements. The first through fourth RF filters may be disposed on respective substrates or, alternatively, be disposed on first and second substrates. The first through fourth RF filters are preferably low order filters.
Other features and advantages are inherent in the RF receiver configurations claimed and disclosed or will become apparent to those skilled in the art from the following detailed description in conjunction with the accompanying drawings.